This invention relates to waste gas treatment and more particularly to treatment of waste gases from production of aluminum chloride containing minor amounts (15% or less) of HCl, COCl.sub.2, and vaporized metal chlorides capable of hydrolyzing to HCl and the corresponding metal oxides.
Halogen-containing gases and vapors, such as phosgene (COCl.sub.2), HCl, silicon tetrachloride, titanium tetrachloride, iron chloride, and aluminum chloride, as well as Cl.sub.2 present in the waste gas from an aluminum chloride reactor must be removed from the gas before it is emitted to the atmosphere even when present at low levels of 15% or less. While it is well known, for example, that silicon tetrachloride will react with water to form a gelatinous precipitate commonly referred to as silica gel, this reaction is not desired and, in fact, it is to be avoided because of the plugging of the pipelines and vessels which can occur upon formation of this gelatinous precipitate.
Lowe U.S. Pat. No. 1,451,399 indicates that if silicon chloride is contacted with a jet of steam at a temperature above the dehydration temperature of silicic acid, the silicon chloride will be hydrolyzed to form silica as well as hydrogen chloride by the reaction: EQU SiCl.sub.4 +2H.sub.2 O=4HCl+SiO.sub.2.
however, in actual practice, it has been found that this conversion at ordinary steam temperatures of about 100.degree. C. results in low reaction rates.
R. F. Hudson in "The Vapor Phase Hydrolysis of Non-Metallic Chlorides" published in Volume 11 of the International Congress of Pure Applied Chemistry London Proceedings in 1947 indicated that this reaction must be carried out at much higher temperatures. He reports that Daubree noted that silicon tetrachloride and water vapor react at red heat in the presence of oxygen to give highly crystallized silica. Hudson then states that as oxygen and silicon tetrachloride do not react until higher temperatures are employed, this indicates a vapor phase hydrolysis at temperatures on the order of 700.degree. C. Hudson then experimentally verified this by reporting experiments conducted in the temperature range of 25.degree.-100.degree. C. wherein no vapor phase hydrolysis of the silicon tetrachloride apparently occurred, at least without the presence of a small deposit of what he termed as "silicon". Hudson then states that he did observe reaction to obtain highly crystallized silica at a temperature of 400.degree. C.
It is also known to produce metal oxides from metal halides in fluidized beds. For example, Hughes et al U.S. Pat. Nos. 3,043,657; 3,043,659; and 3,043,660 are addressed to the production of metal oxides such as titanium dioxide or silicon dioxide by reacting the corresponding chloride with oxygen or air in a fluidized bed at temperatures of 500.degree. C. or higher. Van Weert U.S. Pat. No. 3,642,441 reacts metal chlorides with steam or water vapors in a fluidized bed. However, again the bed is operated at an elevated temperature of 600.degree. C. or higher by the combustion of a gas such as propane in the fluidized bed.
While these reported reactions do, of course, solve the problem of disposal of metal chlorides such as silicon chloride without the formation of undesirable gelatinous products, the proposed temperatures for carrying out the reactions make such processes economically unattractive. Of course, it is recognized that these references had the production of such oxides from concentrated chloride vapors as their objects rather than the purification of gases containing minor or trace amounts of chlorides. It is therefore an object of this invention to provide a process for purification of waste gases including halogenated gases capable of being hydrolyzed by conversion of the gases to form solid metal oxides by hydrolysis without the excessive use of heat. These and other objects of the invention will be apparent from the specification and drawing.